FIELD OF THE INVENTION
The invention relates to a supporting element for an electric winding, a turbogenerator.
A conductor winding configuration for a large electrical machine, in particular for a turbogenerator, is disclosed by European Patent 0 481 984 B1. The conductor winding configuration is constructed in such a way that slots, in which electric conductors are disposed in each case, are provided in a supporting element. The electric conductors are in each case insulated and fastened in the slot by suitable measures. The conductor winding configuration is surrounded by a curable filler, such as a synthetic resin for example, in a total-immersion impregnating process. The initially liquid filler is cured at a temperature between about 100 and 200.degree. C. After curing has been carried out, the conductor winding configuration, which has then combined to form a compact solid, cools to room temperature. In the process, thermally induced shrinkage occurs. That leads to tensile stresses within the configuration. It is thus possible for cracks to form. Corona discharges, which in particular can also lead to damage to the conductor insulation over the course of time, are able to occur at such cracks. According to EP 0 481 984 B1, such crack formation is to be concentrated in a controlled manner in a region in which only harmlessly low electric field strengths are intended to occur. To that end, a semiconducting tape is wound onto the insulation of the electric conductor, forming a first semiconductor layer. A second semiconductor layer is wound over the first semiconductor layer, and is connected electrically to the first semiconductor layer. A separating layer, which is permeable to the filler, is disposed between the two semiconductor layers. That separating layer, which preferably contains a mica flake tape, represents an intended rupture point, so that thermally induced stresses essentially lead to crack formation only at the separating layer. Such a procedure therefore results in a concentrated, controlled crack formation in the region of the separating layer. The separating layer lies in a region of low or vanishing electric field strength, since it is disposed between the two semiconductor layers, which are connected electrically to each other. The two semiconductor layers are at virtually the same potential, as a result of which the region between the two semiconductor layers becomes virtually potential-free and therefore virtually field-free. Damaging corona discharges thus do not occur at the cracks concentrated in the region of the separating layer.
German Published, Non-Prosecuted Patent Application DE 42 19 064 A1 describes a corona shield configuration for a stator winding of an electrical machine. In the illustrated stator winding, an electric conductor which is surrounded by a main insulation is disposed in a slot of a core stack. The slot has a slot wall. The main insulation is surrounded by a first corona shielding layer made of a semiconductive material. A second corona shielding layer is applied to that first corona shield layer. The second corona shielding layer rests on the slot wall. It is treated in such a way that it repels an impregnating resin, in which the stator winding is totally immersed. Therefore, the second corona shielding layer adheres neither to the first corona shielding layer nor to the slot wall. Thermally induced expansions between the electric conductor and the core stack are thus possible to a large extent without mechanical stresses. The formation of cracks, at which corona discharges can occur, is thus largely suppressed.